Pathogenicity of the entomopathogenic fungus Metarhizium Anisopliae (Metchnikoff) Sorokin against the potato aphid, Macrosiphum Euphorbiae (Thomas), on tomato plants

Forty-seven isolates of the entomopathogenic fungus Metarhizium anisopliae collected from soil in Tennessee were bioassayed against the potato aphid, Macrosiphum euphorbiae Thomas, in the laboratory and the greenhouse. In the laboratory, potato aphids were susceptible to all isolates of M. anisopliae and mortality ranged from 43-98%. The six isolates that produced 90% or more aphid mortality were 79.97, 18.97, 44.97, 38.97, 100.97, and 112.97.

The dose-responses of the potato aphids to M. anisopliae were evaluated using selected isolates that caused high mortality in the screening tests. Also isolate 67.97 was included since it was used in other experiments and serves as a standard. This isolate, however, caused one of the lowest aphid mortalities in the screening test. Conidial suspensions ranged from 3.0 x 10⁴ to 1.0 x 10⁷ conidia/ml. The estimated median lethal concentration (LC₅₀) of isolates 18.97, 38.97,44.97,79.97, 100.97, and 112.97 at 6 days post-inoculation were 1.53 x 10⁵, 2.94 x 10⁵, 3.25 x 10⁵, 1.62 x 10⁵, 5.40 x 10⁵ and 3.71 x 10⁵ conidia/ml, respectively, in experiment I; and 4.25 x 10⁶, 3.43 x 10⁵, 1.32 x 10⁵, 8.29 x 10⁵, 8.64 x 10⁵, and 1.84 x 10⁵ conidia/ml, respectively, in experiment II. Isolate 67.97 had an LC₅₀ of 3.30 x 10⁶ and 4.56 x 10⁶ conidia/ml for experiment I and II, respectively. These results confirmed the low virulence of isolate 67.97 toward the potato aphid. The isolates 38.97, 44.97 and 112.97 affected the potato aphid consistently in both experiments. A greenhouse trial to test the virulence of M. anisopliae against the potato aphids was conducted with the three isolates with the best performances in the dose-response experiment. The highest mortality of potato aphids, caused by isolate 44.97 at 1.0 x 10⁸ conidia/ml, was 43.4%. Isolate 112.97 caused 39.6 and 34% mortality of aphid at 1.0 x 10⁸ and 5.0 x 10⁷ conidia/ml, respectively. Poor performance in the greenhouse test may be due to environmental conditions which did not favor the fungus.